The focus of this proposal is to investigate the critical biochemical characteristics of cells that express the oncogene Ras which make them susceptible to stimuli and events that advance the process of carcinogenesis. The proposed research will investigate the novel concept that phosphatases involved in a negative feedback loop that constrains Ras-stimulated signaling are vulnerable targets during early stages in carcinogenesis. We believe that carcinogenic agents disrupt the normal mechanisms by which cells compensate for the presence of activated Ras, and thus unleash super-activated signaling cascades. The role of aberrant regulation of signaling in carcinogenesis has been primarily studied by direct stimulation of signaling pathways. The proposed studies are innovative because they focus on an important alternative mechanism by which signaling pathways can become disregulated. Specifically, the proposed studies are designed to investigate the hypothesis that at early stages in carcinogenesis the activity of Ras-stimulated signaling pathways is inhibited by the upregulation of phosphatases. The subsequent down-regulation of the phosphatases releases Ras-stimulated pathways from negative constraints, which in turn results in aberrant activation of pathways that control cell fate and function. Activation of Ras is a frequent early event in human cancers, including lung and colon cancers, which are two of the most common cancers in the U.S. Thus, there is an urgent need to understand how the presence of activated Ras oncogenes sensitizes cells to agents and events that advance carcinogenesis. Specific Aims 1 and 2 are designed to use a variety of biochemical and molecular techniques to elucidate the mechanisms by which phosphatases that are important negative regulators of Ras-stimulated pathways are modulated in cells that express activated Ras. Specific Aim 3 is designed to investigate the functional consequences of the disregulation of such phosphatases in cells that express activated Ras. The recognition that aberrant regulation of phosphatases is associated with various disorders, including diabetes, obesity, and cancer, has sparked an interest in phosphatases as potentially important therapeutic targets. We expect the proposed studies to provide fundamental information on how disrupting the regulation of phosphatases can contribute to the process of carcinogenesis. Such insight will help in the identification of new diagnostic tools, the development of new strategies to block the advancement of carcinogenesis, and the development of chemotherapeutic agents to treat cancers that are characterized by aberrant Ras activity.